Various proposals have been made for achieving effective marking of substrate, by causing a change of color in or on the substrate, on which the marking is to appear. Further, as indicated below, various thermochromic marking components, i.e. activatable pigments, have been proposed, which may be used to mark a substrate upon application of energy, typically laser energy, i.e. heat.
WO 02/01250 discloses the use of oxymetal salts, such as ammonium octamolybdate (AOM), as marking component in laser marking. AOM is an example of a marking component that can be marked directly with 10,600 nm laser radiation. Laser radiation having a wavelength of about 10 μm may for example be obtained by use of mid-IR CO2 lasers with an emission wavelength in the range 10,000 nm to 12,000 nm. However, mid-IR CO2 lasers are less suitable for installation into existing production lines due to their physical bulk size.
Leuco dyes are marking components that are colorless in one form, but may change into a colored form when exposed to a particular stimulus. Most leuco dyes respond to changes in pH, typically being colorless under alkaline conditions but becoming colored in an acidic environment. Leuco dyes responding to changes in pH, i.e. halochromic leuco dyes, are typically used in combination with a thermal acid generator (TAG). Upon application of heat, the TAG releases protons, thereby the pH is lowered and the halochromic leuco dye is converted into its colored form to provide a marking. A well known TAG is benzylhydroxybenzoate. Other examples of TAG:s are amine neutralized alkylaromatic sulphonic acids, cf. WO 2010/029331, and amine salts of organic silicon or boron compounds, cf. WO 2006/108745. Similar to AOM, also substrates coated with TAG/Leuco dyes may be marked directly with 10,600 nm laser radiation.
An NIR fiber laser may have a small print-head fitted to the production line, connected to the laser, several meters away, via an umbilical cord. Thus, the disadvantage or mid-IR CO2 lasers, being less suitable for installation into existing production lines due to their physical bulk size, may be overcome by use of a NIR (near infra-red) laser and by addition of a NIR-absorber to the ink formulation. The NIR-absorber will absorb near infra-red laser irradiation and convert it into conductive heat. Thus, ink formulations comprising a marking component, e.g. AOM or a Leuco dye and a TAG, and a NIR-absorber may be marked by use of a NIR-laser instead of a mid-IR laser. A common example of a NIR-laser is Nd:YAG laser.
Various types of NIR-absorbers are known within the art and examples of NIR-absorbers in the art comprise:                Organic dye/pigment types. Examples of such MR-absorbers are for example disclosed in U.S. Pat. No. 6,911,262 and WO 2008/050153;        Stoichiometric inorganic pigment types, such as Copper salts, eg. copper (II) hydroxyl phosphate. Examples of this type of NIR-absorbers are disclosed in WO 2005/068207; and        Conductive polymers. Conductive polymers are materials that, in the polymerized state, comprise linked monomers (typically rings) that are conjugated and which can therefore allow delocalisation/conduction of positive or negative charge. Examples his type of NIR-absorbers are disclosed in WO 2005/012442        
Further examples of NIR-absorbers are disclosed in WO 2005/012442, WO 2005/068207, WO 2007/141522 and WO 2008/050153.
A preferred type of NIR-absorber is disclosed in WO 2007/141522. In said publication various types of non-stoichiometric inorganic pigment types are disclosed. Non-stoichiometric refers to the fact that the ratio of elements in the compound may not be represented by integers. A preferred example of non-stoichiometric inorganic NIR-absorber is reduced indium tin oxide (r-ITO).
According to WO 2007/141522, r-ITO, e.g. in the form of a nanopowder, may act as a highly effective absorber of near infra-red radiation in the region 900 to 2500 nm. Thus, it may be incorporated in an ink formulation comprising a marking components, such as AOM or a Leuco dye and a TAG, which upon exposure to radiation in the near infrared region of the electromagnetic spectrum (780 to 2500 nm), provides a color marking, such as black color marking.
However, there is still a need within the art for further improving the printability of ink formulations comprising NIR-absorbers.